I once got off the train after a really long journey feeling pretty rubbish, and as I stepped onto the platform I noticed there was a newt there. I stooped down and picked it up to move it out of the way, but then saw I was (just) too late and it had already been stepped on. It was still moving but only death throes. I felt so bad I cried all the way home and then went straight to bed to cry some more.
At some point before I cried myself to sleep I wondered why on earth I was in such a state, and then I realised my period was due the next day.
Sometimes even when you know your period is due soon it doesn't occur to you that it is affecting your mood until you are already in a complete state.
I think that sometimes people think that if we know we are pre-menstrual, then we should be able to shrug off the irrationality or control it. But that's not how feelings work.